Graphene is a two-dimensional allotrope of carbon, commonly referred to as monolayer graphite. The structure of graphene is a single atom thick planar sheet of sp2-bonded carbon atoms that are packed in a honeycomb crystal lattice. Graphene has various specific properties owning to its ideal two-dimensional structure, including, for example, high conductivity, and high electron mobility. Properties of graphene, such as inelastic electron conductivity and spin transport, mechanical strength, light absorption and emission, and heat conduction, have also attracted interest.
Thickness, lateral size, defects, impurity contents, stacking order, surface chemistry, and edge geometry of graphene are important factors, which determine the superb combined properties of graphene, especially its overall electronic, magnetic, optical and catalytic properties.
At present, various methods for producing graphene are known, including methods, such as, exfoliation of graphite, solid-phase reduction and vapor-phase growth methods. In the solid-phase reduction method, silicon carbide crystals are subjected to a vacuum heat treatment to evaporate the surface Si element, which is called silicon sublimation, and form a graphene crystal layer on a silicon carbide surface. In the vapor-phase growth method, the raw material hydrocarbon gas is fed to grow a graphene film on a crystal surface of a metal, such as nickel or iron, using a thermal chemical vapor deposition (CVD) method.
Most of the carbon sources used in the abovementioned methods, are purified chemicals, which are expensive for mass production. This is a known drawback of the solid-phase reduction and the vapor-phase growth methods. In fact, the CVD method provides large crystalline graphene sheets, but this approach is largely inapplicable to bulk production, because it requires a catalytic thin film made of metallic substrates, such as copper, nickel, molybdenum, etc., which makes this process very expensive, and therefore, undesirable for large scale production.
Moreover, in methods utilizing metal foils, such as the aforementioned CVD method, the amount of graphene, which is produced is limited by the area of the starting substrate/foil. Additionally, chemical vapor deposition on foils has limited application, where a continuous film is required.
Exfoliation or chemical reduction of graphite oxides is another method for producing graphene in a scalable manner; however, the use of toxic chemical agents, as well as complex processing, inhibits the scaling up of such processes. Compared with other techniques, chemical exfoliation, which involves the direct exfoliation of various solid starting materials, such as graphite oxide, expanded graphite and natural graphite, is advantageous in terms of simplicity, cost and high volume production. However, currently explored chemical solution exfoliation methods have a number of drawbacks that need to be addressed. Hence, there is a need for a cost-effective and environmental friendly method with milder operating conditions for mass production of graphene and nanoporous graphene. There is also a need for a method to produce nanoporous graphene with a high surface area, small pore size, and large pore volume.